Bore holes in subterranean formations are often treated to increase their permeability in order to enhance recovery of hydrocarbons. Fracturing of the formations is carried out to increase the production rate of the oil and/or gas contained in the local strata. The fracturing procedure increases flow by creating new fissures and facilitating the connectivity of the existing pores and natural channels contained in a reservoir rock which would otherwise not allow adequate flow to reach the well bore in sufficient quantities for commercial value.
Hydraulic fracturing is a process by which cracks or “fractures” in the adjacent substrate or zone are created by forcing a viscous fluid at a rate and pressure that exceeds the parting pressure of the rock. The continued injection of the viscous fracturing fluid expands the fractures and a particulate material such as sand may be suspended in the fracturing fluid and introduced into the created fissures.
As the pumping pressure at the surface is released, the “frac” fluid will retreat from the formation back to the well. The particulate material, known as proppant, is left behind and acts to prevent the expanded fractures from closing, allowing conductive channels to remain. The viscosity of the fracturing fluid is important for the transport of the proppant material into fractures. Poor or low viscosity can lead to a phenomenon known as “screening out,” where the proppant drops out in the well hole and the penetration of the proppant into the fractures is significantly impaired. After the fracturing process of the formation is accomplished, the fracturing fluid is generally broken, and reverts to a low viscosity fluid for removal from the formation. The viscosity is reduced by breaking down the high molecular weight molecules or by compressing the molecular conformation of the molecules contained in the fracturing fluid. A common approach has been the addition of a slow acting breaker to the fracturing fluid prior to pumping the fracturing fluid into the subterranean formation.
The majority of hydraulic fracturing treatments are water-based fluids. Polymers are typically used to enhance the viscosity of the fluid. Polymers can be classified by the following types: naturally occurring, modified naturally occurring and synthetically derived. Naturally occurring and modified naturally occurring polymers used as viscosity agents frequently include a polysaccharide such as guar, derivatized guar polymers such as hydroxypropylguar, hydroxyethyl cellulose and hydroxyethyl cellulose derivatives. These are economical water-soluble polymers and can be readily crosslinked, which further increases the viscosity of the fluid.
In order to allow gelled and crosslinked polysaccharide fracturing fluids to carry sufficient proppant material, high concentrations of crosslinking agents are often required, which increases the cost and viscosity of the fracturing fluid. Water-based fracturing fluids including gelled and crosslinked polysaccharide gelling agents have had significantly reduced fluid loss as compared to other fracturing fluids, which reduce or eliminate the need for costly fluid loss additives. However, because the gelled and crosslinked polysaccharides have a large molecular dimension due to the linking of individual polymer molecules, the filter cake produced from the viscous fracturing fluid on the walls of well bores penetrating producing formations and in fractures formed therein are often very difficult to remove.
Another problem experienced in the use of a water-based fracturing fluid including a gelled and crosslinked polysaccharide gelling agent is that it must be mixed in holding tanks for a considerable length of time for hydration of the gelling agent to occur. During the fracturing process carried out in a well, the hydrated fracturing fluid generally is pumped out of the holding tanks, mixed with proppant and other additives on the fly and pumped down the well bore to the formation being fractured. If, during the job, the down hole pressure profile and other parameters that are obtained in real time indicate that a change in the fracturing fluid properties is required, that is, a change in the fracturing fluid viscosity to prevent screening out of the proppant from the fracture or the like, it is generally impossible to do so since it takes a very long time for an adjustment to be made and for the changed fracturing fluid to reach the formation being treated. Another problem related to pumping the fracturing fluid from holding tanks and combining the proppant material, crosslinker and other additives used on the fly is that the procedure requires the use of expensive metering and other similar equipment.
In some instances, dry synthetic polymers based on acrylamide are used to enhance the viscosity of the fluid. In general, synthetic polymers are less likely to contain debris and insoluble contaminants than their natural polymer counterparts. Synthetic acrylamide-based polymers are more linear in nature and offer the potential for higher molecular weight when compared to polysaccharide-based polymers and this reduces the dosage required to produce a given solution viscosity. However, dry polymers can be difficult to “make down” into a solution. This difficulty results in the viscosity of the fluid being less predictable, in the formation of “fish eyes” (insoluble or partially soluble polymer particles and agglomerates) and in a general lack of homogeneity in the fluid.
Thus, there is a need in the art for improved subterranean formation treating fluids that have short hydration times, do not produce insoluble residues, do not leave a filter cake, have high proppant carrying capacities and can have their properties changed “on the fly.”